How to calculate step per unit

[Fig]

Hi Quentin,
What’s the drive ratio of X/Y axis of Morgan? I guess it’s 12.0375, right?
Because I’m using 0.9 degree motor and A4988 on X/Y axis, pulleys are 20 teeth (I know you are using 16 teeth), but when I set M92 X215 Y215 machine didn’t work fine.
Could you post the exactly formula of calculating step per unit?
Thanks.
Fig.

[Fig]

And I’m going to design new drive wheels in order to make them more suitable for drive belts. So, drive ratio is important…

[Quentin Harley]

Ratio is important, but you don’t need to know the exact ratio. Steps per angle are calculated during the calibration routine. If it is not possible to reach the calibration position for the first step, increase the step rate and try again.

[Quentin Harley]

PS: you said the machine is not fine. How did it behave exactly on 215?

[Fig]

The routine of hotend is in trouble, when I input G1 X 100, the hotend didn’t move as a line…So, I doubt it was the ratio trouble.
In addition, I use 20MXL pulleys, since the drive wheels you shared have no tooth, I want to design drive wheel with MXL teeth in order to prevent slipping.

[Quentin Harley]

There are no slip if you push the ends of the belt though the holes in the drive wheel, and if not tight secured.

Did it move in a curve even after calibration?

[Fig]

Yeah, but it only moves in curve on X, on Y is in line.
And after I input M114, only current position I can get, but no step cal. How can I get the step cal to input M92?

[Quentin Harley]

So you are not geting something like this?

SENDING:M114
X:170.57Y:65.76Z:235.00E:0.00
SCARA Theta:-0.00 Psi+Theta:120.00
SCARA Cal – Theta:5.86 Psi+Theta (90):16.35
SCARA step Cal – Theta:-0.00 Psi+Theta:573.67

On starting up and connecting, do you get something similar to this?

Connecting…
start
Printer is now online.
echo: External Reset
Marlin 1.0.0
echo: Last Updated: Jul 15 2013 19:44:48 | Author: (qharley, Morgan SCARA)
Compiled: Jul 15 2013
echo: Free Memory: 4849 PlannerBufferBytes: 1232
echo:Stored settings retreived:
echo:Steps per unit:
echo: M92 X426.64 Y430.25 Z105.13 E472.14
echo:Scaling factors:
echo: M365 X1.03 Y0.99 Z1.00
echo:Maximum feedrates (mm/s):
echo: M203 X300.00 Y300.00 Z300.00 E45.00
echo:Maximum Acceleration (mm/s2):
echo: M201 X400 Y400 Z400 E8000
echo:Acceleration: S=acceleration, T=retract acceleration
echo: M204 S9000.00 T3000.00
echo:Advanced variables: S=Min feedrate (mm/s), T=Min travel feedrate (mm/s), B=minimum segment time (ms), X=maximum xY jerk (mm/s), Z=maximum Z jerk (mm/s)
echo: M205 S0.00 T0.00 B20000 X10.00 Z10.00 E5.00
echo:Home offset (mm):
echo: M206 X5.86 Y-13.65 Z0.00
echo:PID settings:
echo: M301 P22.20 I1.08 D114.00

[Fig]

Thanks Quentin, I’ll try again, and post the log in time.
And one more question, I printed new arms which are 100mm longer than Morgan’s you shared. Can I change “Length of inner support arm” and “Length of outer support arm” to fit for my new arms? Will it lead to lower accuracy?

[Quentin Harley]

Write down your calibration settings before you reset. That way you could start on good settings. What are the settings now? You could drop the boot splash here

[Fig]

At SCARA Settings in configuration.h, I want to change both “linkage_1” and “linkage_2” from 15000 to 25000. And at calculate_delta in Marlin_main.cpp, I want to change both “linkage_1” and “linkage_2” from 150 to 250.
Is there anything wrong with my settings?

[Quentin Harley]

That will work fine. Your resolution will go down initially, but you can always print larger drive wheels to compensate, especially now that you have the huge possible build area.

[Fig]

Thanks!
Will gear motors work fine on X/Y axis? If so, I’m going to use them on X/Y axis instead of normal NEMA17, in order I needn’t to print larger drive wheels and still can keep high resolution. Certainly I need to calibrate step per angle as well.

[Quentin Harley]

As long as the gear motors does not introduce too much hysteresis it should be fine. What is the steps per turn on those motors?

[Fig]

I have bought NEMA17 1.8 with 1：14 gear box, which means 2800 steps per turn on its drive rod.

[Quentin Harley]

Yes… That should do it!

I would not bother with larger wheels then.

What is the price difference?

[Fig]

Of course gear motors are more expensive, but my Ultimaker may have no enough area to print larger wheels…

[Author]

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